[clang][driver] Rename DriverOption as NoXarchOption (NFC)

As discussed in [1], ClangFlags::DriverOption is currently only used to
mark options that should not be forwarded to other tools via `-Xarch`
options. This patch renames this flag accordingly and updates the
corresponding driver diagnostic.

A comment in ToolChain::TranslateXarchArgs is also updated to reflect
the change. The original comment referred to isDriverOption(), which is
no longer available.

[1] http://lists.llvm.org/pipermail/cfe-dev/2020-October/066953.html

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D89799